利用者:Sobotka/Blender Image Formats

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Blender Image Formats

Blender supports a number of 2D image formats for input and output. While it might seem like a topic that an artist might feel they need not care about nor research, file formats are the backbone of all work and not all formats are equivalent.

This piece should help the artist learn some of the core concepts behind file formats and their attributes. It will require some background to fully understand the implications and ramifications of choosing a particular format over another.

Why do I care?

File formats are complex creatures and all of your work depends upon them if you intend to store your work. What makes an image format complex covers several broad categories.

What are some concerns I should have when selecting a file format?

While no listing can be entirely complete, the following list of attributes might be worth an artist to evaluate before choosing a file format.

Bit Depth

The amount of data per image unit may be limited by a given file format. An artist seeking archival quality may need a deeper bit depth than a format may allow, for example.

Compression

Compression should be familiar to most artists. Lossless compression will reduce file size while keeping the data as 1:1 within the limits of the format. Lossy compression will discard data in exchange for a smaller and more compressed file.

Color Model

Not all file formats choose the common RGB storage model. Some alternate color models include YCbCr, XYZ, and CMYK. File formats may only allow a specific color model while others may allow for several or many. An artist should be aware of a file formats native color storage model as well as any implied conversions between models that may degrade or alter the data. An artist should not assume that conversions will be lossless nor accurate.

Color Space

While the common color space of the internet is sRGB, there are many other color spaces. File formats may support storing color space data within the format to preserve the intent of the image. An artist should be aware of any implicit color space transformations that may be incurred between formats.

Alpha

Many file formats support the notion of an alpha channel. There are two unique forms of alpha: associated (also known as premultiplied alpha) and unassociated (also referred to as straight or key alpha.) Some file formats permit both forms of alpha, while others may strictly insist on only one.

Crop Regions and Overscan

Some file formats will allow for a granular descriptions of the image dimensions within the file format. In some cases, coordinates and measurements may indicate that the image data is a cropped section of a larger image. In other cases, the data may indicate that it is a superset or overscan region. An artist should determine if a file format supports the features they require for a given task.

Labels and Metadata

File formats may contain mechanisms to specify additional channels over and above the core set to depict a color model. These might include two dimensional data such as a Z buffer or other arbitrary data. In addition to this, some formats permit labeling of channels as well as additional metadata that is of use in certain contexts. Timecode, color model, color space, alpha type, and other such things may be described in this metadata.

Should I be concerned about loading of file formats?

Yes. Given the above series of concerns, some applications may load files as their designers saw fit, which may include irregularities in handling. Further, some file formats will be loaded into systems according to the architecture of the program. This may result in anomalies or inconsistencies with other applications.

The best thing an artist can do is educate themselves and keep on their toes. Testing and knowledge go a long way to avoiding imaging problems.

Are there any nuances or details I should be aware of within Blender that may impact my creative work?

Blender, like any other creative application, has various complexities that artists should be aware of when dealing with images. The following breakdown is a listing that should aid artists when importing and exporting image file formats in Blender.

An artist should pay close attention to the following notes regarding imaging formats within Blender:

• Blender will not properly consider embedded color space information in image formats.
• Blender will not deal with crop regions and overscan in applicable formats.
• Blender will ignore labels and metadata in image formats that support them.
• Blender assumes sRGB gamma for all image formats where a gamma is implied. An sRGB conversion from an 8 bpc source can only be properly stored in a 32 bpc linear format, such as what Blender uses for its internal working space. Any export bit depths less than 32 bpc will suffer degradation.

TIFF

• Bit Depth: Blender supports 8 bpc TIFFs. Blender also has preliminary support for 16 bpc TIFFs.
• Compression: Blender will deal with the compression internally. Blender assumes lossless import and lossless export. An artist should note that 8 and 16 bpc TIFFs are not capable of storing full image data from within Blender.
• Color Model: Blender assumes an RGB color model.
• Color Space: An artist should take care to understand that every import or export of TIFF format will incur an sRGB transformation.
• Alpha: Blender assumes associated alpha for all TIFFs. An artist should be sure to select "Premultiply" and "Color Unpremultiply" for proper compositing of TIFF images.

PNG

• Bit Depth: Blender supports 8 bpc PNGs.
• Compression: Blender will deal with PNG compression internally. Blender PNG assumes lossless import and lossless export. An artist should note that 8 bpc PNGs are not capable of storing full image data from within Blender.
• Color Model: Blender assumes an RGB color model.
• Color Space: An artist should take care to understand that every import or export of PNG format will incur an sRGB transformation.